Prospecting



G. HERZOG PROSPECTING Oct. 20, 1953 8 Sheets-Sheet 1 Filed March 19, 1951 V 5 mm R Z 0 N ER v m K .4 a w. N 0 D .A/ m 0 m r w H P-v m G 0 F M 4 Z m S S v S S S v 0 w a v w m m U U u m m 0 o m o m o o o w N N w W 7 W 3 M m R .GREEQIE a w a FIG. I.

GEIGER MUELLER COUNTER I/SEC.

ATTORNEY G. HERZOG PROSPECTING Oct. 20, 1953 8 Sheets-Sheet 3 Filed March 19, 1951 G. HERZOG PROSPECTING Oct. 20, 1953 8 Sheets-Sheet 4 Filed March 19, 1951 W/ WW i IN VEN TOR. GERHARD HERZOG #6 was m m u v n v M .QSQ \EEQQQ buy Raw mm kw ow 3 x mm NM w kbuhwoku G. HERZOG PROSPECTING Oct. 20, 1953 8 Sheets-Sheet 5 Filed March 19, 1951 :lv bogw a/ H a m m N m w 5 G I F M m 0 0 w. i

VE/N POOR ORE [000$ FIG. 6.

m E m x m w m m H 0 S V m BEwEhs xx SWINGERS 0F ZINC ORE cRossa/f yew-e000 /z/-c & LEAD ORE INVENTOR. GERHARD HERZOG ATTORNEY Oct. 20, 1953 G. HERZOG PROSPECTING Filed March 19, 1951 8 Sheets-Sheet 6 500% ARBOR/vb 115vaoorr sms/nwrrz 77M DELAY 10 SE;

TIME DELAY /.0sC.

2 T/ME DELAY LOSE: SPEED 35 MPH SANDSTONE GRAN/TE GRAN/TESCH/ST SANDSTONE INVENTOR. GER/MRO HERZOG ATTORNEY per gram of rock. It is the relatively slight differences in this radioactivity, particularly gamma ray intensity, at spaced points in the general neighborhood of an ore body which indicate the presence of the latter. As indicated above, the determination of these differences in reasonable observation times and with detectors of reasonable size, requires the use of detectors having a high gamma ray counting efficiency, several times the efiiciency of the conventional Geiger- Mueller counter consisting of a tubular cathode around a wire anode. Such conventional counters have an efiiciency of not to exceed about 70; i. e. they detect on the average only one ray out of 209 received. At this low efiiciency significant diiferences in gamma ray intensities may well be obscured. Gamma rays are emitted sporadically and at random and unless the observation time is long enough the counttaken at any point may not be representative of the source. Moreover, even though the counting efiiciency be increased or the time made long enough to counteract this tendency to error, significant differences may be obscured by accidental variations including those due to cosmic rays.

To state my discovery in another way, I have determined that it is entirely feasible to discover ore bodies by a method involving the detection of gamma radiation intensity, even though the bodies themselves are buried by overburden so thick that no practicably measurable amount of radiation emanating directly from the ore body penetrates to the surface at which the measurement is taken and even though the ore body be one which contains no material amount of radioactive minerals and would generally be classified as non-radioactive. The invention, in this aspect, is based upon my discovery that the substantially barren country rock or overburden which effectively prevents significant amounts of radiation emanating from the body from reaching the surface, in many cases manifests suflicient variations in gamma ray emissivity to act as a marker for the ore body. In other words, even though the ore body itself be substantially non-radioactive or if radioactive, so deeply buried that its emanated rays do not penetrate to the surface in significant amounts, gamma rays emanating from the barren ground (in which the ore body occurs) penetrate to the surface and will, by proper detection, yield an intensity pattern which reveals the ore body. In short, the invention contemplates the improvement in prospecting for deeply buried mineral deposits which comprises locating a related radioactivity anomaly at a remote earth surface by accurately determining the gamma ray intensities along said surface, the term deeply buried being employed herein to mean that the deposit is so far removed from the points at which the gamma ray intensity measurements are made that there is no significant variation in the measured intensities due to gamma rays emitted directly from the deposit.

In a series of investigations conducted in a number of mining districts throughout the western United States, including Butte, Montana; Bisbee and Miami, Arizona; Cripple Creek and Climax, Colorado; the Coeur DAlenes, Idaho; Grass Valley and Idria, California; and the Tintic and Bingham Canyon areas in Utah, it has been determined that useful gamma ray anomalies indicative of a variety of types of mineral deposits may be discovered through the practice of the invention. These anomalies have been found where the country rock in which the ore body occurs is igneous and also in cases in which the country rock is sedimentary. The reasons for the radioactivity of the country rock in the neighborhood of the ore body are notcompletely understood and may be various, depending upon themanner in which the ore body was formed and its subsequent'alteration. The naturally radioactive elements include those of the uranium, thorium and actinium families and potassium. In some instances, radioactivity in the country rock adjacent to an ore body may be due to introduction of one or more of these elements at the time the ore body was formed, say by magmatic intrusion. In other cases, it may be due to radon, actinon, or thoron which penetrated the. rock subsequently either in gaseous state or in solution in water. Whatever be the reasons, the radioactivity of the country rock and the variations in this radioactivity are suificient to enable discovery and delineation of associated ore bodies in a great variety of cases.

Theoretically, perhaps, a gamma ray detector of any efficiency might be employed for the detection of deeply buried deposits through the detection of gamma ray anomalies originating in the overburden, provided that a suflicient time were allowed for each reading. With ordinary detectors however, this time is so long as to be completely impractical, and if less than the required time is taken, the readings will be without significance. As already noted, the conventional Geiger-Mueller counter consisting of a tubular cathode disposed around an anode wire has a detection efficiency for gamma rays of approximately /2%. At this efficiency, the time required for each observation becomes a matter of hours and even then the results may be vitiated by accidental variations. Consequently, from a practical standpoint, a detector having a considerably higher counting efiiciency for gamma rays only, say a crystal type of detector, should be employed.

Any type of gamma ray detector may be employed for the discovery of auras of mineral deposits through the practice of the invention, provided that the gamma ray intensities to be measured are sufiiciently high and differ greatly from point to point along the traverse being prospected, and provided further that sufiicient time is employed for each reading. In the majority of instances, however, the gamma ray intensities to be measured (and more especially the significant differences in gamma ray intensity to be determined) are small, so that special equipment is desirable.

Generally speaking, ionization chambers are less desirable than tube counters, for example Geiger-Mueller detectors, in the practice of the invention. In turn, conventional Geiger-Mueller counters are not as suitable as other types of detectors which have a higher counting efficiency for gamma rays and substantially the same efiiciency for all other types of radiation. Thus the crystal type of detector (employing a diamond or some other gamma ray-sensitive crystal as a detecting element) or detectors of the type described and claimed in U. S. Patent No. 2,397,- 0'71; granted March 19, 1946, are presently preferred.

Since, in general, the gamma ray intensities to be measured are of a low order, counting rates tend to be small. The counting rates may be increased by employing a single Geiger-Mueller detector of large size, i. e. by increasing active omega counter yoluine, butzcounting: rate asiwelhasiemciency of detectiommay beincreased by increasing cathode area in theGeiger-vlueller detector, for exampla by employing a ibundle 1 of :Geiger- Muellerrietectors prefera'tbly disposed-in a single envelope -=a-s -is disclosed, for =exam-ple, in Patent No2i48644 issued November 1, 1'94-9 to D. :G. CJHare) Othen factors being equalp it is better to *employ -a-'nurn-ber of small Gieiger -Mueller detectors connected in parallel than :a single such detector of 'equiva-lentactive volume, since in this way the advantages of increased-cathode area and increased active volume are obtained.

;.As-.-;indicated .above, :a Geiger lyiueller .;detector :consists essentiall of l tubular (cathod an coaxial wire anode running-.throughiit. Thetwo areienclesediin-an envelop -within which a.- ui eee u a os here (say-a .miXtl I arson and alcohol) jsanaintained, usually at ,a sub.- iatmospheric pressure. Normally the .potential difference between the cathode .,and {the .anode .isjnearlyfbut vnot quite, highenoughto cause a-, discharge to take place. ,If an ionizing .ray -passes into the detector adischargemay take .placewith-resultant current flow. ,The discharge ceases. after a-.-short period of time, .after -which the. counter .is again incondition to register .or

count .iionizins way 'rEor .all --.except ga ma .radiation, the Geiger- Mueller;counter islhighlygefiicient. It will detect alpha and beta-radiations and the-penetrating particles .oiflcosmimrays zyviths substantially 100 'emcienc-y. :However, its efliciency ;for gamma radiation is low, .say=o .1y. /2% oronthe average only one out .of each 1:200 gamma rays entering its -aetive volumeitrig erstthe-counter. We have discoveredthatltor the .location in :a surface serof. theiaintsdifierences in gamma ray in- .tensities that are indicative of. most-deeply buried .ore-gdeposits, as-wvell as most eontactsifa-ults, and similar geolo i f es th su ey rshouldr made with a detector which has a substantially higher efiiciency forgamma rays than the conventional Geiger-Mueller detector (preferably at least 4 or 5 times) and substantially the same or no greater efiiciency than the conventional .GeigerrMuellQr :counter for call other :radiation. Ionization chambers do not serve :-.this;pnrp'ose, but crystal type detectorsiwhich, employ a crystal, such as a 'diamon'd'that becomes momenitarily. conductive 'upon the entry of: radiation and permits the passage of current), detectors-emplaying a fluorescent screen of napthalene to obtain scintillations which are measured by a photomultiplier tube and-detectorsofthemulti ple cathode plate type,. .as described and claimed 'inU. S. Patent No. 2,.:-97;0'71, granted March 19, ,1946, are wellsuited to-the..prac tice ofthisaspect oflthejinvention. Thus myinvention contemplates the conduct of a survey alongor above the earths surfacebmmeasuring the gamma ray intensities at aseries of points on or above the surface with --an efficiency substantially greater than-that'obtainablewith a conventional Geiger- Mueller counter ,whilesimultaneously measuring together with the'gamma radiation intensity. the intensity of otherradiation present but with substantiallyithe same or no. greater efficiencythan that obtainable -.with the conventional Geiger- ..Mlleller counter. ,{I-n'this vwayiaint-difierences ,ingarnma ray intensity which-areindicative-of geological. features such-as tanks-contacts; minzeralized r zon s. 1 etc, :may :be .detected \tvith Lian yborne instruments oat aelevations up etc 158,1; ileast '6 80.0 crfeet iabove 'athe rearthrwhiie zinovin'e et ih'i h velocity.

-A2fur.ther explanationeofitheradvantages of-ith employment -:in suriace surveys. :and especially surveys. con-ducted .'-at..:substantia1 :elevatiomabove the surface, of :a detectorrhaving :theicharacteristics :describemabove as given hereinafter.

aThesee-and other aspects of ;the invention -:will be clearly understood inuthe ;1ight;of-.;the f ol1ow- :in detailed description, :taken :in moninncticn .with 2theaaccompanyingmrawingsii which i-Fig. 1 is ;a graphical .icom-parisonof (counting rates of -.a conventional :GeigereMueller' ;counter and .a preferred: counter for lthe :practicenfnthe invention;

Fig. 2 r is --:a;rgraphici representation of :a surface survey :made ;in :accordance :with ithe iinvention over a: buried copper v.iclepositiof *the massivlei; fd-isseminated. type rFJig. 3 is arsimilarsrepresentation of :a:-surface surveymade over ;.a1series ofiburiedzreplac'ement type: lead -zinc deposits;

:Eig.-:4;is::a;graph;showing' .the results ofaa? horizontal underground survey'iin 'a limestone :cut -by.;porphyry --dikes and -;a zinc replacem-ent :type :deppsit;

:Eig. 1.5 is 1 a :graph .of :the results cof I an .underground survey galong 5a, @crossecut through .porphyry; sills and lead'zinc-repla-eement deposits .occurringinquartzite;

Fig. 6 shows the results aof .ian -.underground survey through granite. out: byiarzinc-coppertvein indicated by a positive anomaly and 12a zinc=lead vein indicated by a negativeianomaly;

Fig. 7 illustrates a strong positivevanoma'lyiencounteredin anenniierg'ronnd surveyiacross azinc .vein-occurringinrgranite;

-Fig.:'8 givesa;comparison-ofesurvey results with airborne equipment 'sflown atz-various elevations across :a granite :plug :occurr-ing insandstone;

;Fig.?- 9.;gives: a;comparison ofrzsurveyi results with equipment flown at various elevations. andzcarried along the-surface aoross -the granite plug Of Fig. 8;

IFig. 10 illustrates the efiect of *a radioactive survey marker employed -to correlate survey results with terrain-,in.ainborneoperations at various altitudes;

Fig. 1 1 I is a iiia gram illustrating a 3 preferred mounting arrangei-nent for aQbatteiy-df detectors employed in -a surface survey with Fa 5 vehicle or *the like yand Figs. 12 and 13 *show modiiications of the mounting arrangement ofFig: l 1

Substantially any radiation "detector which sensitive to gamma radiation will also detect '--alpha --and beta radiation :a-nd "other radiation -such as the penetratin'g particles ofcosmicradi- --'ation. In fact, -detectors in general have a higher efiiciency-'for'othentypes of radiationthan -for gamma rays. 'ln geop'hysical;prospecting employing radiation deteetors;'-the gamma radiation is detected along with otherradiation, some of --which originates in'the detector itself because of contamination of the materials employed in ..the:construction or the ;d te tor and some .of which is 'cosmicfradiation;which has components so penetrating that ithey -are'iiencountered j'in mines, etc several thousand ieetbelow. the s rface. "Hereinafter, the detected radiation other :than gamma ra diation'is referredto .as fbackground, d i ba k r nd becaus .it is emittedsporadically and at random and also because of its relatively high intensity may well obscure significant variations in gamma ray in- 7 tensities between .diiferent. pointsvalong .a traverse and under the most favorable conditions reduces thecontrast between readings obtained at different points. I have. discovered that with certain types of radiation detectors the efiiciency for detection of background does not increase proportionally to the increased efficiency of the detector.v for gamma rays, with the result that as the detection efificiency for gamma rays is increased, agreater contrast between. readings is obtained-'andcthere is less chance that small but significant variations in gamma ray intensity 'from point to point along or over the earths surface will be vitiated by variations in background, particularly cosmic rays. two of these certain types of radiation detectors are'the crystal type and the type employing a multiplicity of cathode plates disposed transverse to one or more anode wires which pass through apertures in the plates. The virtues of a detector of this type over a conventional Geiger-Mueller counter have been demonstrated in comparative tests. The results of these tests are illustrated in Fig. l, which is a plot of counting rates obtained wtih a conventional Geiger-Mueller counter at a series of points along traverses on the earths surface against the counting rates obtained at the same points with a multiple plate type of counter having 12% less active volume due to minor differences in length and diameter. a

The observed counting .rates of the multiple platetype of counter (hereinafter called the T counter) iarexplotted as ordinates against the counting rates with the Geiger-Mueller counter plotted as abscissa. 7

All the points fit very closely to a straight line relation. The straight line, which is shown in Fig. 1, was calculated with the method of least squares. If T and G are the counting rates per second for the T counter and for the Geiger Mueller counter respectively the equation for the straight line is:

Theoretically one should expect a relationship between T and G according to Equation 2.

' T--'t:K(Gg) (2) where t and g are the constant background rates for the T counter and the Geiger-Mueller count:- er, respectively. The difference T minus 22 on the left side of the equation is then the counting rate due to the gamma rays from the ground alone. Similarly the difference G minus g is the counting rate due to gamma rays alone for the Geiger- Mueller counter. Equation 2 expresses that the gamma ray count for the T counter is a multiple of the count for the Geiger-Mueller counter. Accordingly, K is the relative efficiency to gamma rays of the T counter as compared to the Geiger;- Mueller counter. Equation 2 can be rewritten in the form of Equation 3.

The experimental relation as expressed in Equation 1 has the same form as has Equation '3 and by comparison it appears that the relative efficiency of the T counter compared to the Geiger-Mueller counter is 4.945. The difference 9 minus t divided by K must be equal to 10.843. By substituting the letter C for the value 10.843 in Equationl, one arrives at Equation 4.

As already indicated,

.of the Geiger-Mueller counter.

8 This equation can be written as follows:

t=K(g--C') I (5) showing that the background t for the T counter is not equal to a multiple K of the background g Onehas to subtract the value C from g and then multiply with the efficiency K in order to get the background .t.

The T counter had a volume which is 12 pe rcent smaller than the Geiger-Mueller counter. This changes the constants of Equation 1 to those in, Equation 6. l

T=5.54(G.9.97) V (6) or in other words the relative efiiciency is approximately K =55 and the constant C is equal to 10 counts per second.

The background counts t and g each consist of two parts, which shall be indicated by an index 0 and an index 0. to and go shall be that part of the respective backgrounds which is caused by gamma rays. These gamma rays are partly due to contamination of the metal parts of the counters. It is proper to assume that the number of contamination gamma rays is equal for the two counters. Because of the higher efliciency of the T counter in detecting gamma rays the background count due to the gamma rays to will be K times go. The second part of the background to and go is due to directly; ionizing particles such as penetrating particles of cosmic rays and of alpha rays which'are emitted by the metal. Since the cosmic ray contamination for the T counter would be somewhat less than for the Geiger-Mueller counter due to the smaller Combining Equation 5 with 'Iand 8 one 'obtains Equation 11.

te+tc=K(go+gcC) (11) By inserting values 9 and 10 into Equation 11 one obtains Equation 12.

. X c=K(go+geC) (12) and finally the relationship 13 By inserting the numerical values for K and 0 into 13 one obtains for go the value 12.2 counts per second.

gc=12.2 cts./sec. (14) This figure then represents that part of the background for the Geiger-Mueller counter which is due to directly ionizing rays.

From cosmic ray measurements it is known that the counting rate due to cosmic rays is approximately one count per minute per square centimeter of counting area. The counter in question had a cross sectional area of 700 square centimeters. One may expect, therefore, a counting rate of approximately 700 counts per minute due to cosmic rays alone or a counting rate of 11.5 counts per second. This value agrees very well with the one which was calculated from the field observationsand which is reportedin Equation 1.4,- andv supports. quitestrongly the validity of thecalculations made above.

The foregoing calculations. show plainly that the background of the T counter (which is described more'fully in U. S. Patent No. 2,397,071.)- does not increase proportionally to the increased efliciency for gamma rays. This is emphasized by the followingtabulationof counting rates-it 55x G=I15 170. 225 280 335 The second line gives the observed counting rates with the Geiger-Mueller counters for the counting rate which islistedinthe first-line. for theT counter. Inthe thirdline are the-values which would be obtained. if. K:5- /2., Geiger.- Mueller counters were used simultaneously. From the first and the last column itis apparent that the counting rate forv the T. counter increases by a factor 5, whereas the counting rate for 5. /2 Geiger-Mueller counters increases only by a factor of 3. In other words, let us assume that there are two regions in the. field, one of which showsa. low counting rateand-the. other one a high-counting rate- In the first: region the T counter gives a; rate of 50: and in the second region. a. rate of 250 counts. per second; The T counter, thereforeshows a contrast bya factor of 5; byusing simultaneously 5 Geiger-Mueller counters each. one of which has thesame active volume as. the. '1. counter one. finds in the first place a counting;- rate. of 1 15 and inthe. second location one of 335 counts per second. The aggregate or the 5 Geiger-Mueller counters, therefore, indicates acontrast which. is slightly less than 3. This indicates clearly that. the T count er has advantageswhich cannot be made. up by using a large number of Geiger-Mueller: counters.

In undergroundgamma. ray surveys, .the cosmic ray. effect and hence theback-ground,. is diminished substantially by absorption in. the: over.- burden. However, in-surface-surveys (as shown by Fig. 1) the cosmic rayeffect. is highand in airborne surveys; the background; due to. cosmic rays is even higher, and the tendency to obscure slight differences in gamma ray-intensities from point topoint greater; tendency may; be

overcome: and satisfactory results obtained by employing a detector. of the: type just: described; or any other type which. has a substantially higher efiiciency for gamma. rays than; thereonventional Geiger-Mueller detector and in which this increased efiiciency; is obtainedwithout-im and, thev Geiger-Mueller counter; are considered as they afiect the useable portion of the; total counting rate observed, it is readily apparentswhy the results obtained with the T counterare. more accurate than those. obtained with. the. Geiger..-

Mueller counter, even though a, longer observa- If one now measures the same locations tion .time is employed using theGeiger-Mueller counter,

Assume the applicationofaT. counter and a Geiger-Mueller counter. of equal active. volumes and a reading. time for each which will. result in the same. number of total countsobserved. This will give readings. which are subject to the. same probable statistical error. as applied to theaverage counting ratesv observed. However, due. to the lower efiiciency for gammarays. of the. Geiger- Mueller counter, this probable error. is a much larger. percentage of. the. useable counting rate (the'observed. counting rate with. the background subtracted) than. is the case with the. higher efficiency T counter where the useable. counting rate is armuchlarger portionof. the total. count.- ing. rate observed..

The invention, as applied. to. the. locationof buried. mineral" deposits. through the discovery ofradioactive. anomalies. associated with the country rock,. contemplates underground;. surface, and airborne operations employing. any: type of detectorv of. requisite size and efliciency. Estpeciallyin underground operations where: the gamma. ray intensities may be. relatively high, Geiger-Mueller counters. are. sometimes useful. In. surface and airborne surveys for the. detection of faint. anomalies characteristic. of. some. min-.- eralized. zones as. wellas some faults, contacts, etc.',.i. e. when the inventioniis'applied.asan aid in geological-mapping, the .inventionicontemplates the use. of. the high eificiency counters already described.

Either positive. or. negative. gamma-ray anomalies may characterize a buried-ore-body.. Inrsome cases the country. rockisimorerradio activerthan the ore body anctas the latter is approached the intensity of detectedgammaradiatiom decreases. This. is. considered. a negative. anomaly. the. reverse case. inwhich gamma ray intensityine creases-as theorebody-is=approaohed being considered. a-.positive anomaly.

A- positive: anomaly discovered-in a-survey' of a deeply buried disseminated copper deposit in Arizona is illustrated in Fig. 2; Inthis survey a single. multiple plateemultiple wire detector was. employed The envelope was: a-v gas tight brasscylinder 15 long'and 3. in.-diameter.. The cathode plates were silver andlwere. groundedto the envelope. There were 65: such; plates,. each .010" thick and'spacedon centerswith their major surfaces perpendicular to: the cylindrical axis. Seven tungsten anode-wires (.005" dia were passed through holesiin the plates-.- At each station-the detector was laid approximately flat on the ground and 8000 counts were-takem the-average probable statistical error for. such a length of count being-1. 2

In all cases' except stations '7?- and 8 wheres the top: or: a liill had been flattened'with. a. bulldozer; the stations were on substantially undisturbed ground, i; e; bedrock.

Considerationof the plot of gammarayintensities, beginning at theright, showssubst'anv tiallyuniform and relatively lowval'ues over-the schist. The'contact'of the schist with the granite porphyry is shown by an increase in intensity; Save for the readings at stationsfland 8 (which may be lessreliable than the others due tocthe disturbed. surface), the intensities increase markedly as the depth to the ore. body'decreases with a strong increase in intensity at the. left wherethe ore: body is. close to:the surface. It

will: also be apparent that. the: detector distinguished between the granite and the granite porphyry of the overburden.

Fig. 3 is a vertical geological section through a mineralized zone in Colorado with a graph of intensities observed at a series of stations on the surface above the section. In this, work a single 3" diameter by 15" long multiple platemultiple wire detector (similar to that used in the survey of Fig. 2) was employed. The ore bodies underlying the traverse are sulphide replacement deposits occurrin along bedding planes in limestone. The limestones are interbedded with sandstones, grits and mudstones and there are several post-mineralization faults as shown. The rocks of the section are covered throughout by glaciofiuvial gravels of varying depth and at no point is there a surface exposure of the ore. The survey was begun on the uphill side of the section (the left side in Fig. 3). The first station was above unmineralized limestone overlain by gravel and a low intensity level of about '70 counts per secondwas re istered. The second station is still to the left of the first ore body but the count at this point is substantially higher. The next four stations overlie the upper lead-zinc sulphide deposit, which is overlain at one point by a deposit of pyrite and pyrrohotite. All of them are points of high intensity, as is the next station (No. 6). Having in mind that the ore of this first body is nowhere closer to the surface than 10 feet and is over 100 feet below the surface at station 6, it is plain that the detector is beyond the range of detectable differences of gamma radiation ori inating in the ore body. This indicates the presence of a radioactive aura in the country rock overlying the body.

A second ore body of lead and zinc sulphide underlies the surface at the ri ht of the section by depths in excess of 200 feet. Its presence is indicatedby an increase in gamma ray intensity at stations 9, 10. 11, 12 and 13 which overlie it, the unmineralized zoriebetween the two bodies being characterized by low intensities of stations 7 and 8.

Fig. 4 shows the results obtained in a horizontal underground gamma ray survey conducted along a crosscut in limestone in an Arizona mine. Three different limestones were crossed, as shown. A replacement deposit of zinc sulphide ore occurs in one of the lime formations and another one is cut by porphyry dikes. A marked aura occurs in Limestone A in the neighborhood of the dikes, i. e. thereis a marked increase in gamma ray intensity from this lime at a substantial distance, i. e. in excess of 50 feet, from the dike.

An even stronger positive anomaly is associated V with the zinc deposit which occurs close to a contact between the Limestone B and Limestone C. This anomaly is detectable at a distance of almost 200' feet to the left of the deposit inLimestone B in which it occurs but is less evident to the right, possibly because station No. '14., at which the intensity is relatively low, occurs almost-on the contact between Limestone C and Limestone B. In other words, the contact or interface may have retarded the migration of gamma ray sources toward the right.

The survey illustrated in Fig. 4 was conducted with a 3" x 15'' multiple plate multiple wire detector similar to that used in the survey of Fig. 2, the points of highest intensity being checked with a smaller multiple plate single wire detector 2" in diameter and 4 long. The detectors were stationary and substantially vertical in the center of the cross-cut at each station.

Fig. 5 illustrates an underground survey in a Utah mine in which lead-zinc sulphide deposits have replaced thin limestone beds in quartzite, with porphyry sills intruded along the bedding planes and generally parallel to the ore bodies. In this work a single small multiple plate detector was employed (2" diameter, 4" long) with 12 lead plate cathodes and a single coaxial tungsten anode wire. The porphyry sill, as might be expected of an acid intrusive, shows a marked positive anomaly, its aura in the quartzite being apparent at least 40 feet away. The vein on the left likewise is accompanied by a strong positive anomaly which is apparent in the quartzite at a substantial distance, indicating again the presence of an aura. Having in mind that in many cases exploration openings have missed valuable ore bodies by a matter of a few feet, the ability to locate an aura of a deposit, and thus in effect see through the intervening rock, is an extremely valuable aspect of the invention.

Figs. 6 and 7 illustrate actual underground gamma ray surveys made in accordance with the invention in a Montana mining district. In these surveys a single detector was employed with a metallic envelope 2" in diameter and 10 long. It was provided with 51 silver cathode plates spaced as in the case of the apparatus employed in the survey of Fig. 2 and wth a single coaxial anode wire of tungsten. The surveys were made along crosscuts about 2000 feet underground. In both cases the country rock was granite.

To consider Fig. 6, the gamma ray intensities beginning in the granite at the left are relatively high. In the neighborhood of the slight stringers of zinc ore a marked negative anomaly is encountered. Immediately thereafter however, the country rock to the left of the zinc-copper vein gives a high intensity which increases markedly as this vein is passed and then drops to the intensities observed in the unaltered granite. Continuing to the right, another sharp negative anomaly appears in the neighborhood of the zinc-lead vein.

The relationship of the stations at which the readings were taken to the position of the veins is shown at the bottom of Fig. 6. From this relationship it will be plain that the anomalies appear in the country rock well outside the veins.

Fig. '7 illustrates a traverse across a zinc vein which shows a marked positive anomaly. This anomaly is apparent in the country rock on both sides of the vein, as well as in the vein itself and illustrates clearly a situation in which the detector can apparently see the vein through several meters of granite, although what is actually being detected is the aura in the country rock, in this case granite, which lies adjacent to the vein.

One of the important fields of application of the invention is in surface reconnaisance, the detector being moved along the traverse continuously with continuous integrated observations. An efiicient detector together with the associated power supply, preamplifier, amplifier, integrating circuit, registering meter, etc., with a total weight of less than 40 pounds has been constructed, and this unit may be carried by an observer walking along the traverse, a con tinuous indication of gamma ray intensities along the traverse thus being obtained. Larger detectors having a higher counting rate may be truck mounted or airborne and anomalieshave been detected while moving the detector falong the traverse at speed up to 2 06 miles per hour and at elevations up' tl3=-more than one-thousand feet.- 1

All things being equal} greater contrast/= is olitaix'ied, i; e2 thedeteotionof significant axiom alies is more pronounced; if i the: detectors are carried J along the traverse at: a substantial dis-.- tancg abovathe-ground; improvement in result being: obtained as the distance is increasodl up to ai-h'eight ofabout lIfeet; above .which results become: less satisfactory: and more difficult to interpret;

number;v of? comparativei surveys have: been made in which the: detectorsiwere frrstsmounted in a truck and runralong the=traverse andithere aften flowmacross: thel traverse; in" as helicopter at difierenttael'evations; In this work six large multiple plate-multiple \viredetectors werezemzployed.v Each. was 3: inches. in; diameter and}; 30 inch-ess'long 'andtthey vvere mounted-rside a by side tmforml a panel: Each detector" contained 136 silver cathode platess .0111 thickr mounted on centers and 7: tungsten"; wire: anodes-z (00.51

dial) passingz"perpendicularlythroughz die ametenholes -inzthe plates; Each detectorrwas separately preamplifiedz. and) quenched, the out;- puts1 of thezseveralpreampiifiers bei'ngtsenh to am" electronicsmixing circuit. which: turn. fed aisingle amplifienz The; output: Off this amplifier was integrated; and; recorded-by an: electronic voltmeten The integrator employed was: so designed? that; its-1 time' constant; i; e.:. tha'time ovenwhichztlie input'apulses;were.;averager could be-wariedi.

the airbornez surveys. as: will. presently" ap pean from: a; discussion: ofi 8 andi'iksit. was found: that; optimum: results; were obtained while flying? at. w an elevation; off 100; tor 20.0 feet: above groundi. aheight on about; 15.0;feet: being? best. Extremely useful results however: wererobtained uprtotheights of 800: feet whiles flyinaz over anomalies of: relatively low; gamma; ray intensity}: These effects of heights uponi result; appear to be* due: to: thee interplay of) avnumhen of factors: viz;-

lz. Au lowelt-itudes-abovei grounded; e: loser-than about '75 feet; slight. (inferences; in? the-:altitude; encountered; for. example, when: theinstrument iszcarried oven at ditch; -cause marked: deviations recorded intensity: Since these: deviations are notz related ten the: geological features being investigated, they; tend to. abscureiresultsr Above about: -7.5 -feet, theseeffects: are: less? pronounced and; hence: there is less-requirement for-the pilot toj attempt to: fly at constant elevation; above grounds 2.. As, height is increased; cosmic, ray; backs groundincreases andthe intensities ot-gamma raysrfromthe .groundl decrease; so=that at above; says 800 feet, significant changes: in gamma ray intensity due tosgeological. difierencess-toabe discoveredi-tendto be obscured-1unless-theyare very proncunceda As altitude; increases,- the -path of thevgamma rays;.-throug h=the air lengthens.. This in turn.- reduces the intensity due to.--absonptioni by the air.

3; As-heig-htis-increased; gamma ray intensity from any given point in the earths crust devcreases according to: the; inverse i square law, but atrlthei-same times-the area from-.rwhicl'n the detect.- tor receives gamma. radiation increases. These factors: oppose? each:- other; so; that the: inverse square; law; not: thGiSUlGTODIflIIOHiHgS factoni 4. ..Atlow-elevations; radiatione emanatingzdi'om a buried source off to one side offthezdetectonhas toz-passv through. a relatively long path; in high density overhurdemwithi consequenmhighrabsorpfitioniandim'arked decreasein'intensity: Aszelevas tioniabove ground 'islincreased ther'path through the; overburden:decreases;..the net result; being thatrthis; radiatiorr:mayi have: aagreaterreflectton the :measurement; even though: the-:totali' distance that the; radiation travelszth'roughz ground and air. (which: because of its: 1ow density..- has: low absorptiv'epower isincreased;

In thezlighuofi the: foregoing, it. wouldiappeare that-even .whenzthe detector; is carrieialon'g'itlie surface s at"; substantially; constantdistancerabove ground byzranimal; on vehicle',-p and evemwhen' readings: are made above; the surface: with the deteotorg'stationary; advantages accrue to: in:-v creasing' the": elevation of the. instrument; above ground: Hence .it is better. tomountathe: detector. above a.truckzratherzzthani'below and stillhettei: tomount. the; d'etectorfso: that; screening efiect of; thetruckzisminmized:

Several. preferred: mounting: schemes: for groundbornezdtectors are -illustrated in Figs; 11 to 13.

Referringi-to Figs lilsiit willsbe seen thati a-spanel of multiple;plate-multiple anodes? detectors? are mounted." on a. support?v above: a; vehicle; say'ian automobilawwith theirrmaioraxis: (astdefinedrby the anodes of thetdetectors) substantiallypare all'elr to: the direction? OE' travel? off the r vehicle: They' are. mounted sufiiciently' high a bove'. -the vehicle, say 3:01: 4ifeet;'.that11theabsorptioniefieot of'the vehicletonfgammairaysemanatingfromzthe surrounding terrainiis reducedL-WhiIetheigmHnd area from which:thecdetectorsireceiveigammairar diations' is. increased; The detectors i are so; disrposed that their maximum cross sectionalzarea zis disposed: approximately. parallel: to: the" pathi of travel and i'n-"a: substantially: verticalrplane; .so. that they interceptv gamma radiationi front the sides'ofithetraverse.

Eig: 1'22 illustrates: aimodification; of the are rangement ofiFlig. III. in which; the'adetectors ;are broken; into two: separate. panels; off three? each disposedmespectively onrthe'two sidesrof the vehiucleFbut*eleotricallyi'connectedras already described withi separate preamplifi'ersz feeding; a: common mixers.

Fig:v illustrates ranotlier" modificationtofe the arrangement: of? Fig; 12: instwhich .-th'e acres ofvthe detectors. are, vertical;'. the? arrangement being othenwise theisamer.

three ofi the foregoing: arrangements are advantageous in sunveyss conducted overundiss turbed;==surfaces=.andialsotaiong established roads, wherewfiils, .pavementg-tetov, .mago tend: to; obscure results. They permitiincreased 1 reception: from undisturbed terrain along: the? side: of: the: road andith'us-t'end toiimprove reliability of result.

A l-comparison .01? results obtainediwithzautomotbileemountedsand:ainborneidetectorsemployedito traverse'.- a;- gramte': p'l'ug" occurring sandstone in 'I exas'i-is given iniFig:.-9. Thelowestitrac'e on this figure was obtained withithe -.p&n1 0f SiXf d8-.- tectors mounted horizontally: oni-"th'e' 'roof? of: an automobile? drivem a lonmthef traverse? at; ai speed 011 about 35': anilesr-pei 'houir. The. othertraces were obtained l witl'ntliet'same'. panel-- of: oiedsect'cn's mounted inca helicopter; an'dzflown along-itheztrav; erseiatztl'i'e elevations and speeds indicatedifor each-trace? In all -casesithe ftime 'delay? i2 e..the integration? constant of: the: integrating circuit interposedr between the amplifier and? the re corder was the same, so that the results aredirectly comparable.

, To consider the lowest trace, obtained with the detectors mounted on the automobile and beginning on the right, it will be seen that the contact between the sandstone and the granite and schist occurring on the right of the plug is marked by a substantial increase in intensity of gamma rays. The contact between the granite-schist and the fresh granite is less marked but is distinguishable as a small negative anomaly, Whereas the contact between the fresh granite and the sandstone at the left of the figure is marked by a sharp decrease in intensity as the detector passes toward the sandstone. This trace shows clearly the pronounced effect of surface features on the detector at low elevations. Thus when the detectors were carried across a bridge over a dry creek bed, so that their distance above ground increased a few feet, there was a sharp drop in intensity. The survey which the lowest trace represents was made along a paved highway, and it is to be expected that better results might be obtained over undisturbed ground, although not as good as those obtained with the airborne instruments.

The trace obtained when the detectors were flown across the traverse at an elevation of about 100 feet has the same general shape as that of the trace taken immediately above ground level. In

representative of geological features are easier to detect. For example, at 100 feet the contrast between granite and sandstone and that between granite and granite-schist are substantially greater.

The trace obtained at 150 feet above ground is still better than that obtained at 100 feet, despite the fact that the speed of travel was considerably greater (i. e. 60 M. P. H.). Thus the trace is, in general, smoother, insignificant variations'being minimized while the indices. of the contacts between rocks are more easily recognized. So, although the trace has the same general shape as those taken at the lower elevations, the several formations are more sharply defined and there is less hash on the trace.

At an elevation of 200 feet, as shown by the upper trace, the several formations are still sufficiently defined to be identified. However, contrast between formations has in part been lost,

and the traceis notso easy to interpret as that made at 150 feet above ground.

Fig; 8 continues the comparison of traces made with airborne detectors at different elevations across the same granite plug surveyed in Fig. 9. The detection equipment and the aircraft were the same as in the operations illustrated by Fig. 9, but a different time delay, 1. e. integration constant was employed in recording the traces. Surveys were run at elevations of approximately'100, 200, 400 and 800 feet above ground at approximately the same air speed M. P. H.) Thefour traces thus obtained are shown. All of the traces follow the same general pattern, but as elevation is increased minor and insignificant variations tend to be removed. The traces at 100 and 200 feet, show the greatest contrasts for the several geological formations and features to be identified, but differences between the sandstone and the fresh granite, between, the granite and the granite-schist and between the granite-schist and the sandstone are still apparent at 400-and 800 feet. In short, Fig; 8 shows that the optimum altitude for airborne gamma ray surveys in accordance with the in- Vention is between about and 200 feet,.but that useful results, even with relatively low intensity anomalies, can be obtained up to 800 feet. In still other tests, flown in an airplane as dis: tinguished from a helicopter and at much higher speeds and elevations, high intensity anomalies (such as a shore line where there is a strong contrast between gamma ray intensities emitted by land and Water) were detected clearly. 800 feet, therefore, is not necessarily an upper, limit, of usefulness of the invention, although it is, well above the optimum elevation of about feet, at which height the effects of deviations in elevation above ground are less pronounced; and less care need be taken in maintaining constant this elevation.

From the foregoing discussion of Figs. 8 and 9, it will be plain that by continuously integrating the pulses as a function of time and adjusting the integration time constant in relation to the speed of movement of the detector over the surface being investigated it is possible to emphasize contrasts in gamma ray intensities due vto geological changes (say at a contact between different rocks) or geological differences in: the earth underlying the line of movement of the detector while suppressing contrasts in gamma ray intensities due to surface features (say the presence of a ditch). For any given speed of movement of a detector at a given elevation, there is an optimum integration time constant, and the invention contemplates the adjustment of time constant in relation to speed of detector movement or height above ground or both, in order to secure maximum contrast between significant gamma radiation along the line of detector movement, either at substantial elevations or at the surface. I

It will also be plain that for a given integration time constant, the speed of movement of the detector over the terrain should be kept sufficiently low that differences in gamma ray intensities due to geological changes in the earth underlying the line of movement of the detector are not obscured by the integration.

In airborne surveys conducted in accordance with the invention it is essential that the traces obtained, say those of Figs. 8 and 9, be correlated with the terrain over which the survey is made. This may be done in several ways.

One simple way is to provide an automatic time marker in the recorder, so that the time each portion of the trace is recorded is known. The operator of the detector notes the exact time that the aircraft is directly above prominent landmarks along the traverse, or above monuments or markers disposed on the ground along the traverse, and thus is' able to correlate surface with trace by means of time.

Another, but more complicated method, is to employ a camera which is geared to the recorder of the detector. In this way a series of pictures of the underlying terrain is taken as the traverse is flown simultaneously with the recording of the trace of radiation intensity and correlated therewith. a

A third method involves the use of radioactive markers which are disposed at selected points along the traverse and are registered on the trace itself. It has been established, as'shown in Fig. -10, that a 5mg. source of radium gives a sharp pip? on .the' trace atzelevationsiiupg to at:least-v 160 feet; These. pips-,2 shou-ldgthey happen to coincide; withzagamma; ray anomaly in the ground, may be confusing. In orderrto avoid possibilityof such confusion it is; desirable to? repeat the survey without. employing; the radioactive. markers: and comparing; the two traces thus obtained.- Another way to avoid confusion isto employ; two radioactivesmarkers spaced from each-other far: enough toproduce two pips onthe; traceaand thus produceea char= acteristic and easily recognizablepattern.

Referring to Fig, 10, the trace"A", was; obtained while fiyingat an. elevation of.20 feet at aspeed of about 30 miles perhour-over apoint at which a 5 mg. source of rad-ium wasisubse-i quentlyplaced, the detectors employedbeingthe panel of 6' previouslydescribed; The .fiight vwas repeated at an elevation, of. 20- feet (trace B;.") with the. radium source in" place and: a .pronounced pip, was obtained, so.- pronouncedydn fact; that it ran offi'the. scale. Additional re-z peat runs were made-atelevations respectively of 40,,80, 160, 300 and.400.feet (traces 15. and G). The pipisppronouncedaup to 160 feet, andirecognizablezeven ,atAOO feet.

A 5- mg. source ofradiumis extremely-small, again indicating-the surprising naturewof the re,- sults' obtainedand: the. high efilciencyof the multiple-plate .typeofj detector for gamma. rays.

easily with a. multiple:platersinglewire counter of the type' described in. connection withFh'g. 5.

The aspect of the 1 invention illustrated in: Fig, 10 (which involvesdetermining the location. of an aircraftwith respect to terrain over, which it is flying; by detectingfrom;the-aircraft the increasezin gamma radiation from a. source, such as a small quantity of radium, located ata known ppint'on-the, terrain) is useful outside of the prospectingfield, and-is, in fact,,of gen,- eral utility in blind :flying. Thus the position of an airfield oranyother'groundinstallation may be. marked by placing, radioactive. markers: at known locations; say at the: four," corners of, a: field. A gamma ray: detector "of relatively high efficiency is mounted in an aircraft, and a strong.

pip? will be indicated :by; the :recorder-or.in-.- dicator of :the'detector when the: aircraft is over. In order; to distinguish one monu.-- ment' or marker from another,- the-size ofrtha radioactive sources employed may bedifierent, or a monumenttmay; consistof a plurality; of. separate sources spaced far. enough apart .in a horizontal directionthat eachproduces a-;sub-.- stantially separate pip, thus: producing at the;

the source.

recorder a distinctive .pattern. For example; one.

cornerof afield mayi bemarked with one source:

a second with two,-.etc., ora distinctive. pattern may be produced by employing as a monument atplurality of spaced sources of different sizes. The: height at which the" gammaaray point: source is detectable is a function of they-size As.

shown in Fig. 10,. a very small source-iseasily detected at heights in excess of two hundred.feet,-.

Air navigation, through the application ofia. system of radio-active markers (saypoint sources.

ofradium) .at known ground locations, offers outstanding advantages, first becausethe. emission.

of gamma rays by the marker. is. assured, where-.

optimum: height for. observation." of; gamma ray intensities; in airborne: surveys .-conducted fin. accordancewith myginvention; local. conditions may require a vdifferent .heigl 1-t for. maximumv contrast, and the-invention: contemplates-broadly theaprace tice;of-1; fiy ing-,;a detectoracross terrain of :an ,elevationlabovei grcundsuch that maximum cone trast is tobtained-zbetween. gammaaradiationfrom theeearth and other: detected; radiatiom. Stated otherwise, thep-detector'isipreferably; moved over the, surface-of theeart-h;at-.. a; height above-sand belovewhich .-there-?is at decrease: in .the -contrast between the intensity; of the; detected gamma rays -.from.:the-;earth: and: the other detected radi ationtii e., background-3. Preferably this dee tectionismade continuously and thG-iCOUI'SGgfiOWIL is'f at-.- least; approximately;' a, constant distance above ground.

Surveys made accordance withmymventi-on havenshowmthat. aagiven :typesof occurenceiina iven lccal-ity (say aacoppfinvein-uin graniteiin. one :mining district .whereethe -.conp er. veins have been-:formedaunder;similamconditionssand some,- timesw-at about.-,the@-same-.:time) willLmanifest-a typical; anomaly; In. such. case, it advanta=egeous'. to.-=survey ,-one orsmoreaj known deposits.;.todetermineathe rtypical .-anomaly;, and; to look; for. similar, anomaliesrinw the-neighborhood whichare not associatedawith known, deposits,.-,-but; which: may;revealyoneehitherto undisclosed-'1' This :isaan. important ,feature of: theinventionandone which tendsito'reduce thetprospecting risk-isubstantial-- ly.-- It isr nctanecessarytthat.thegknowncdeposit; which-. issunveyed-gbe intact at the-time it lSrSllI- veyed, for it z hast been a demonstrated; that, the anomaly remains after; thee-deposit; has: been: mined; out-,r ,in.-wholezorg;in-,.parti Consequently. a. survey; inathe(neighborhood;of;:a deposit which has; been; removed? may, establish the: type of, anomaly to v beasoughtz in. surveys. conducted; in: themeighborhood: ,ito .-locate extensions; of pthe ;de-.-- nositworysimilar new .occuntences.

Asapplied; toethe-r d'lSCOVGIY' Off anomalies indie cated: ;by; difEerences.;-in thezintensity. of; gamma;-

rays 2 emitted from-.1diiferentpoin-ts- .--in; thel-.oyere burden of a mineral deposit, the invention contemplates surface: and underground operations,

thesterm.surfacebeingemployed toinclude those.-

conducted on'the surfaceaasi well-as above the surface; i.; c: with; airborneinstrumenta. The discovery-of;the-anomalies in underground.operations may -be; madeby surveys conducted along boreiholes or.-:development openingssuch as:raises; and crosscuts.

When employing;countersehaving;asubstan=- tially higher: efficiencythan-conventional Geiger-- Mueller counters butin-which the counting ef ficiency forbackgrounddoes not increase proportionately to the increaseiin gamma ray counting a efiiciency, theinventionuhas more: generalappliecation in" surface-operations and may-be used-"not: only: to discover such anomalies :butralso. as: an:

aid in geological, mapping, i.- e. inthe discovery of, contacts, etc. I The-method .mayrthuszrbe used to cover a. large area rapidly; and; cheaply with: airy borne instruments. The results obtained will permit accurate geological mapping of the terrain, but in the normal case most of the area will yield only negative results from the standpoint of locating mineral deposits, i. e. no substantial anomalies will be observed. In those portions of the area in which anomalies are observed in this large scale reconnaissane, detailed investigations should. follow, either with airborne equipment or on the ground using a detector which is carried by man, animal or vehicle.

An anomaly, once it is detected (say from the air) may be outlined by travelling over it (and preferably flying over it) repeatedly along different courses, which may be laid out in the form of a grid. In this way points of equal intensity may be located over the surface and mapped. A convenient way of outlining the anomaly is to join points of equal gamma ray intensity with lines, which may be called isoradins. ily, the isoradins associated with a mineral deposit will be closed loops, which have the appearance of contours around a hill or a depression. If gamma ray intensity increases toward the center of the region enclosed by an isoradin, the anomaly is a positive one. If it decreases toward the center of the region enclosed by the isoradin, the anomaly is negative.

One way of quickly outlining the horizontal proportion of a mineral deposit associated with a radioactive anomaly is by altering the source of the aircraft to fly along the line (isoradin) at which radiation intensity (say intensity of gamma radiation) is substantially constant, marking the course of the flight by aerial photography of the ground over which the plane passes, or by dropping markers, such as sacks of flour.

The isoradin or isoradins may be considered as a projection on the surface of the earth of the outline of the mineral deposit associated with the isoradin, and the invention contemplates the evaluation of mineral deposits by making an aerial radio activity survey, as described above, to obtain the outline of the deposit as a projection on the surface of the earth and then drilling test holes within the outlines as projected on the surface to determine the thickness of the deposit. In this fashion the shape of the deposit in three dimensions is determined. The mineral concentration along the source of the bore holes may be determined by actual sampling, as with cores, or if desired, the same result can be obtained in many cases by logging the holes by passing a radiation detector along them and correlating determined intensities with depth in each hole.

1' claim:

1. In prospecting, the improvement which comprises measuring the gamma ray intensities at a series of points along and a substantial distance above the surface of the earth area being prospected with an efficiency substantially greater than and detecting together with the gamma radiation the intensity of background radiation present but with an efficiency no greater than that obtainable with a Geiger- Mueller counter having an efficiency no greater than one-half percent for gamma rays.

2. In prospecting, the improvement which comprises measuring the gamma ray intensities at a series of points along and from about 100 to about 200 feet above the surface of the earth area being prospected with an efficiency substantially greater than one-half percent and detecting together with the gamma radiation the intensity of background radiation present with an Ordinarefiiciency no greater than that obtainable with a Geiger-Mueller counter having an efficiency no greater than one-half percent for gamma rays.

3. In prospecting, the improvement which comprises measuring the gamma ray intensities at a series of points along and about feet above the surface of the earth area being prospected with an efiiciency substantially greater than that obtainable with a conventional Geiger-Mueller counter and detecting together with the gamma radiation the intensity of background radiation present but with an efficiency no greater than that obtainable with a Geiger- Mueller counter having an efficiency no greater than one-half percent for gamma rays.

4. In geophysical investigations involving the continuous movement of a pulse type detector for gamma radiation over the surface of the earth at a substantial height above it and the continuous detection of the pulses produced in the detector by gamma rays emanating from the earth surface, the improvement which comprises measuring the intensity of gamma rays through the pulses produced in the detector with an efiiciency substantially greater than one-half percent and measuring, together with the intensity of the gamma rays, the intensity of other radiation present but with an efiiciency no greater than that obtainable for such radiation with a Geiger-Mueller counter having an efficiency of no more than one-half percent for gamma rays, integrating the detected pulses as a function of time, employing a selected integration time constant in the integration, and keeping the speed of movement of the detector sufiiciently low that differences in gamma ray intensities as indicated by the integrated measurements and due to geological differences in the earth underlying the line of movement of the detector are emphasized and not obscured by the integration.

5. In geophysical investigations involving the continuous movement of a pulse-type detector for gamma rays over the surface of the earth and the continuous detection of pulses produced in the detector by gamma rays emanaing from the earth surface, the improvement which comprises detecting the intensity of gamma rays through the pulses produced in the detector with an efficiency substantially greater than one-half percent and detecting together with the intensity of the gamma rays the intensity of other radiation present but with substantially an efficiency no greater than that obtainable for such radiation with a Geiger-Mueller counter having an efficiency no greater than one-half percent for gamma rays, integrating the detected pulses as a function of time, and adjusting the integration time constant in relation to the speed of movement of the detector over the surface so as to emphasize contrasts in gamma rays intensities as indicated by the integrated measurements and due to geological differences in the earth underlying the line of movement of the detector.

6. In geophysical investigations involving the continuous movement of a pulse-type detector for gamma rays over the surface of the earth at a substantial height above it and the continuous detection of pulses produced in the detector by gamma rays emanating from the earth surface, the improvement which comprises detecting the intensity of gamma rays through the pulses produced in the detector with an efficiency 21 substantially higher than one-h al f percent and detecting together with the intensity of the gamma rays the intensity of ether radiation present but with substantially an eificiency no greater than that obtainable for such radiation with a Geiger-Mueller counter having an efilciency no greater than one-half percent for gamma rays, integrating the detected pulses as a function of time, and adjusting the integration time constant in relation to the height of the detector above the surface of the earth :so as to emphasize contrasts in gamma rayinten=-- sities as indicated by the integrated measurements and due to geological differences in the earth underlying the line of movement of the detector.

7. In geophysical investigations involving the continuous movement of a pulse-type radiation detector for gamma rays over the surface of the earth at a substantial height above it and 1 the continuous detection of pulses produced in the detector by gamma rays emanating from the earth surface, the improvement which comprises continuously detecting the intensity of gamma rays emitted from the earth through the pulses produced in the detector with a detection efiiciency substantially greater than while also detecting together with the gamma rays the intensity of other radiation present but with substantially an efficiency for such radiation no greater than that obtainable with a Geiger-Mueller counter having an efiiciency no greater than one-half percent for gamma rays, integrating the detected pulses as a function of time, and adjusting the integration time constant in relation to the speed of movement of the detector over the surface of the earth and the height of the detector above said surface so as to emphasize contrasts in gamma ray intensities as indicated by the integrated measurements and due to geological differences in the earth underlying the line of movement of the detector.

8. In geophysical investigations involving the continuous movement of a pulse-type detector for gamma rays over the surface of the earth and the continuous detection of pulses produced in the detector by gamma rays emanating from the earth surface, the improvement which comprises detecting the intensity of gamma rays emitted from the earth through the pulses produced in the detector with an efiiciency substantially greater than one-half percent and detecting with the gamma rays the intensity of other radiation present but with an efficiency no greater than that obtainable for such radiation with a Geiger-Mueller counter having an efficiency no greater than one-half percent for gamma rays moving the detector over the surface of the earth at a height above and below which there is a decrease in the contrast between the intensity of the detected gamma rays from the earth and the other detected radiation, integrating the detected pulses as a function of time, and adjusting the integration time constant in relation to the speed of movement of the detector over the surface of the earth so as to emphasize contrasts in gamma ray intensities as indicated by the integrated measurements and due to geological differences in the earth underlying the line of movement of the detector.

9. In geophysical investigations involving the continuous movement of a pulse-type detector for gamma rays over the surface of the earth at a substantial height above it and the continuous detection of .pulses produced in the detector by gamma rays emanating from the earth surface, the improvement which comprises continuously detecting the intensity of gamma rays emittedvfrom the earth through the pulses produced in the detector with a detection efficiency substantially greater than while also detecting together with the gamma rays the intensity of other radiation present but with no greater efificiency than that obtainable for such radiation with a Geiger-Mueller counter having an efficiency no greater than one-half percent for gamma rays and while moving the detector over thesurface of the earth at a height at which contrasts in gamma ray intensities due to geological differences are at a maximum, "integrating the detected "pulses as a function of time, :and vmaintaining the :time constant of ih tegrati'onin such relation to the speed of movement of the detector over the surface of the earth that contrasts in gamma ray intensities as indicated by the integrated measurements and due to geological differences in the earth surface-underlying the line of movement :of the detector are emphasized.

10. In geophysical investigations involving the continuous movement of a pulse-type detector for gamma rays over the surface of the earth at a substantial height above it and the continuous detection of pulses produced in the detector by gamma rays emanating from the earth surface, the improvement which comprises continuously detecting the intensity of gamma rays emitted from the earth through the pulses produced in the detector With a detection efficiency substantially greater than one-half percent while also detecting together with the gamma rays the intensity of other radiation present but with no greater efficiency than that obtainable for such radiation with a Geiger-Mueller counter having an efficiency of no more than one-half percent for gamma rays and while moving the detector over the surface of the earth at a height at which contrasts in gamma ray intensities due to geological differences are at a maximum, integrating the detected pulses as a function of time. and maintaining the time constant of integration in such relation to the height of the detector above the surface of the earth that contrasts in gamma ray intensities as indicated by the integrated measurements and due to geological differences in the earth underlying the line of movement of the detector are emphasized.

11. In geophysical investigations involving the continuous movement of a pulse-type detector for gamma rays over the surface of the earth at a substantial height above it and the continuous detection of pulses produced in the detector by gamma rays emanating from the earth surface, the improvement which comprises continuously detecting the intensity of gamma rays emitted from the earth through the pulses produced in the detector with a detection efiiciency substantially greater than one-half percent While also detecting together with the gamma rays the intensity of other radiation present but with no greater efliciency than that obtainable for such radiation with a Geiger- Mueller counter having an efliciency of no more than one-half percent for gamma rays, and while moving the detector over the surface of the earth at a height at which contrasts in gamma ray intensities due to geological differences are at a maximum, integrating the de tected pulses as a function of time, and maintaining the time constant of integration in such relation to the speed of movement of the detector over the surface of the earth and the height of the detector above said surface that contrasts in gamma ray intensities as indicated by the integrated measurements and due to geological differences in the earth underlying the line of movement of the detector are emphasized.

12. In geophysical investigations involving the continuous movement of a pulse-type detector for gamma radiation over the surface of the earth at a substantial height above it and the continuous observation of the pulses produced in the detector by gamma rays emanating from the earth surface, the improvement which comprises measuring the intensity of gamma rays through the pulses produced in the detector with an efiiciency substantially greater than one-half percent and measuring together with the intensity of the gamma rays the intensity of other radiations present but with an efliciency no greater than that obtainable for such radiation 24 with a Geiger-Mueller counter having an efficiency of no more than one-half percent for gamma rays, integrating the detected pulses as a function of time at a selected integration time constant, the speed of movement of the detector relative to the surface of the earth being correlated to the selected time constant so that difierences in gamma ray intensities as indicated by the integrated measurements and due to geological differences in the earth underlying the line of movement of the detector are emphas'med and not obscured by the integration.

GERHARD HERZOG.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,397,071 Hare Mar. 19, 1946 2,499,489 Goldstein et a1. Mar. 7, 1950 OTHER REFERENCES Geophysical Exploration, Heiland, Prentice- Hall, 1940, pages 883-885.

Transactions of A. I. M. M. E., vol. 164, Geophysics, 1945, pages 117-124. 

1. IN PROSPECTING, THE IMPROVEMENT WHICH COMPRISES MEASURING THE GAMMA RAY INTENSITIES AT A SERIES OF POINTS ALONG AND A SUBSTANTIAL DISTANCE ABOVE THE SURFACE OF THE EARTH AREA BEING PROSPECTED WITH AN EFFICIENCY SUBSTANTIALLY GREATER THAN 1/2% AND DETECTING TOGETHER WITH THE GAMMA RADIATION THE INTENSITY OF BACKGROUND RADIATION PRESENT BUT WITH AN EFFICIENCY NO GREATER THAN THAT OBTAINABLE WITH A GEIGERMUELLER COUNTER HAVING AN EFFICIENCY NO GREATER THAN ONE-HALF PERCENT FOR GAMMA RAYS. 